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How Light Bulbs Watch You Buy Groceries

In an enormous grocery store in northern France, the lights above the aisles aren’t all they seem to be. They look ordinary—more than a mile and a half of fixtures exuding bright light, folded into a grid overhead—but they’re actually flickering faster than the human eye can see. The unique patterns each individual section of lighting emits are a 21st-century twist on Morse code, meant not for people, but for the cameras on their phones.